Handover (HO) refers to movement of a mobile station (MS) from a wireless interface of one base station (BS) to a wireless interface of another BS. Hereinafter, a handover procedure in a general IEEE 802.16e system will be described.
A Serving Base Station (SBS) in an IEEE 802.16e network may broadcast information about neighbor BSs through a mobile neighbor advertisement (MOB_NBR-ADV) message in order to inform an MS of information (topology) about basic network configuration.
The MOB_NBR-ADV message includes system information about a serving BS and neighbor BSs, for example, a preamble index, a frequency, a handover optimization possibility, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
The DCD/UCD information includes information of which an MS should be aware in order to exchange information through downlink and uplink. For example, the DCD/UCD information includes handover (HO) trigger information, and medium access control (MAC) version and media independent handover (MIH) capability information of a BS.
A general MOB_NBR-ADV message includes information about neighbor BSs of an IEEE 802.16e type only. Accordingly, information about neighbor BSs of a type other than IEEE 802.16e may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. As a result, an MS may acquire information about BSs of a heterogeneous network by requesting a serving BS to transmit the SII-ADV message.
A procedure for an MS having information about neighbor BSs obtained through the above-described method to perform handover in an IEEE 802.16e network will be described in more detail with reference to FIG. 1.
FIG. 1 illustrates an example of a handover (HO) procedure which can be performed in an IEEE 802.16e system.
Referring to FIG. 1, an MS exchanges data with a serving BS (SBS) at step S101.
The SBS periodically broadcasts information about neighbor BSs to the MS through a MOB_NBR-ADV message at step S102.
The MS may start scanning for candidate handover (HO) BSs using a handover (HO) trigger condition while communicating with the SBS. The MS requests the SBS to perform a handover procedure by transmitting a handover request (MOB_MSHO-REQ) message when a handover condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded at step S103.
The SBS informs candidate handover (HO) BSs included in the MOB_MSHO-REQ message that the MS has requested handover through a handover request (HO-REQ) message at step S104.
The candidate handover BSs take action for the MS having requested handover to transmit information about handover to the SBS through a handover response (HO-RSP) message at step S105.
The SBS transmits the information about handover, obtained through the HO-RSP message from the candidate handover (HO) BSs, to the MS through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information necessary to perform handover, that is, a handover action time, a handover identifier (HO-ID), and a dedicated handover (HO) code division multiple access (CDMA) ranging code at step S106.
The MS determines one target BS (TBS) among the candidate BSs based on the information included in the MOB-BSHO-RSP message received from the SBS. The MS then transmits a CDMA code to the determined TBS to attempt ranging at step S107.
The TBS receiving the CDMA code may inform the MS of success or failure of ranging and physical correction values through a ranging response (RNG-RSP) message at step S108.
The MS transmits a ranging request (RNG-REQ) message for authentication to the TBS at step S109.
The TBS receiving the RNG-REQ message from the MS transmits system information, which can be used in a corresponding BS, such as a connection identifier (CID) to the MS through a ranging response (RNG-RSP) message at step S110.
If the TBS successfully completes authentication of the MS and transmits all update information, the TBS informs the SBS of success or failure of handover through a handover completion (HO-CMPT) message at step S111.
Next, the MS exchanges data with the TBS which has performed handover at step S112.
The HO procedure for use in the IEEE 802.16m system is similar to the HO procedure for use in the IEEE 802.16e system. However, individual messages may be called in different ways as necessary.
MOB_NBR-ADV→AAI_NBR-ADV: Corresponding message may include not DCD/UCD formatted system information but S-SFH formatted system information.
MSHO-REQ→AAI_HO-REQ
BSHO-RSP→AAI_HO-CMD
RNG-REQ (CDMA code)→Ranging preamble code
RNG-RSP (ranging status)→AAI_RNG-ACK (ranging status)
RNG-REQ (MAC message)→AAI_RNG-REQ
RNG-RSP→AAI_RNG-RSP: Corresponding message may include a station identifier, such as a TSID or STID, instead of a CID.
Because of channel condition deterioration in the above-mentioned HO procedure, the MS or the SBS may unexpectedly lose an HO-related Medium Access Control (MAC) management message (e.g., AAI_HO-CMD message) while attempting an HO action. In this case, the MS is unable to attempt HO to the TBS and at the same time is unable to receive scheduling information from the SBS after the lapse of a predetermined time.